1. Field of the Invention
This invention relates to hydrocarbons having reduced levels of mercaptans and a method and composition for preparing the hydrocarbons having reduced levels of mercaptans. This invention particularly relates to treating hydrocarbons with a mercaptan scavenging composition to reduce mercaptan levels in the hydrocarbons.
2. Background of the Art
Mercaptans, a source of “sourness,” can be present in both petroleum crude oils and in the fuels produced therewith. These compounds, having the general formula:R—SHwherein R is an alkyl or other organic group, are generally undesirable as they have a very unpleasant smell and can have a negative impact upon the environment. For example, in Manitoba Canada, federal and provincial regulations set 0.5 parts per million (0.5 ppm) as the permissible concentration in the air.
Petroleum and other crude oils are most commonly converted into finished products in a fuel products refinery. Typically, the fuel products produced are gasoline, distillate fuels such as diesel and heating oils, and bunker or residual fuel oils. Atmospheric and vacuum distillation towers are used to separate the crude into narrow boiling fractions. A catalytic cracking unit cracks high boiling vacuum gas oil into a mixture ranging from light gases to very heavy tars and coke. In general, very heavy virgin residuum having average boiling points greater than 1100° F. (593° C.) is blended into residual fuel oil or thermally cracked into lighter products in a visbreaker or coker.
Overhead or distillate products in the refining process generally contain very little, if any, hydrogen sulfide, but may contain sulfur components found in the crude oil, including mercaptans. However, substantial amounts of mercaptans and other organo-sulfur compounds are found in vacuum distillation tower bottoms, which may be blended into gas oils and fuel oils. It is for this reason that mercaptans are often present in bunker fuel oils.
For the purposes of the present invention, “oil” is meant to include the unrefined and refined hydrocarbon products derived from petroleum or from liquefaction of coal, both of which contain sulfur compounds. Thus, the term “oil” includes, particularly for petroleum based fuels, wellhead condensate as well as crude oil which may be contained in storage facilities at the producing field and transported from those facilities by barges, pipelines, tankers, or trucks to refinery storage tanks, or, alternatively, may be transported directly from the producing facilities through pipelines to the refinery storage tanks. The term “oil” also includes refined products, interim and final, produced in a refinery, including distillates such as gasoline, distillate fuels, oils, and residual fuels.
Refined fuels must be brought within mercaptan specifications for marketability. In the processing of oils, it is desirable to eliminate or reduce atmospheric emissions of noxious mercaptan or other organo-sulfur compounds associated with sulfur containing oils, in order to improve environmental air quality at refineries. It is also desirable to remove or reduce mercaptan is fuels, particularly the bunker fuels. For example, large ships, a major user of bunker fuels, can have difficulties with environmental regulations at some ports.
Efforts have been made in the part to sweeten fuels that had sulfur compounds present. EP 0 538 819 to Roof, et al., discloses that Sour sulfhydryl group containing oils can be treated with an effective amount of a sweetening, hydrogen sulfide vapor reducing quaternary ammonium compound. The compounds disclosed in this reference are stated to be especially suitable for high boiling, heavy residual fuels under low mix conditions. EP 0 067 036 similarly discloses Quaternary ammonium hydroxides as mercaptan scavengers.
WO-0234863 to Munson discloses another method of removing mercaptans from hydrocarbon streams. In this reference, it is disclosed to use basic metal salts that react with mercaptans to form mercaptides. The metal salts are dissolved or suspended in ionic liquids, which tend to have virtually no vapor pressure. After the mercaptides are adsorbed into the ionic liquid, the demarcaptanized hydrocarbon stream can be removed, for example by distillation, decantation or gravity separation. Then the mercaptides can be oxidized, for example, by exposure to air, to form disulfides. The disulfides are insoluble in the ionic liquids, and can be readily removed.
Another reference is this art area is EP 0 499 743 to Frame, et al., which is also U.S. Pat. No. 5,064,525. Therein, it is disclosed to use a two step process for sweetening a sour hydrocarbon fraction containing tertiary mercaptans and primary or secondary mercaptans. In one step the mercaptans in the sour hydrocarbon fraction are reacted with hydrogen in the liquid phase and in the presence of a selective hydrogenolysis catalyst to selectively hydrogenolyze the tertiary mercaptans. In another step, the primary and/or secondary mercaptans are oxidized by reacting them with an oxidizing agent in the presence of oxidation catalyst and a basic component.
An older reference in this art area is U.S. Pat. No. 3,144,403 to Jacob. Therein, a process for reducing the mercaptan content of hydrocarbons by oxidation of mercaptans to disulfides is disclosed. In this process, hydrocarbons containing mercaptans are contacted with a phenylene diamine inhibitor sweetening agent and oxygen in the presence of an alkaline catalyst consisting essentially of a solid anion exchange resin. The process further includes separating the hydrocarbons having reduced mercaptan content from the solid resin.